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 A319009 Numbers k such that the multiplicative order of 2 modulo k is psi(k), psi = A002322. 1
 1, 3, 5, 9, 11, 13, 15, 19, 21, 25, 27, 29, 33, 35, 37, 39, 45, 53, 55, 57, 59, 61, 63, 65, 67, 69, 75, 77, 81, 83, 87, 91, 95, 99, 101, 105, 107, 111, 115, 117, 121, 125, 131, 133, 135, 139, 141, 143, 145, 147, 149, 159, 163, 165, 169, 171, 173, 175, 177, 179, 181 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Numbers k such that the multiplicative order of 2 modulo k is at its maximum possible value. Numbers k such that the binary expansion of 1/k has period psi(n). Numbers k such that A002326((k-1)/2) = A002322(k). This is a generalization of A167791, so A167791 is a proper subsequence. Write k as k = Product_{i=1..t} (p_i)^(e_i) where p_i are distinct primes. If (p_i)^(e_i) belongs to A167791 (and thus here) for 1 <= i <= t, then k is also here, but the converse is not true. In fact, this sequence has terms such that none of (p_i)^(e_i) belongs to A167791, the smallest of which is 301 = 7*43. The multiplicative order of 2 modulo 7 and 43 are 3 (< psi(7) = 6) and 14 (< psi(43) = 42), so the multiplicative order of 2 modulo 301 is lcm(3, 14) = 42 = psi(301). LINKS Robert Israel, Table of n, a(n) for n = 1..10000 EXAMPLE The multiplicative order of 2 modulo 15 is 4 = A002322(15), so 15 is a term. The multiplicative order of 2 modulo 21 is 6 = A002322(21), so 21 is a term. The multiplicative order of 2 modulo 51 is 8, but A002322(51) = 16, so 51 is not a term. MAPLE select(n -> numtheory:-order(2, n)=numtheory:-lambda(n), [seq(i, i=1..1000, 2)]); # Robert Israel, Sep 12 2018 PROG (PARI) forstep(n=1, 200, 2, if(znorder(Mod(2, n))==lcm(znstar(n)), print1(n, ", "))) CROSSREFS Cf. A001122, A002322, A002326, A167791. Sequence in context: A173263 A285141 A329095 * A294427 A141231 A287344 Adjacent sequences:  A319006 A319007 A319008 * A319010 A319011 A319012 KEYWORD nonn AUTHOR Jianing Song, Sep 07 2018 STATUS approved

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Last modified May 15 06:18 EDT 2021. Contains 343909 sequences. (Running on oeis4.)